The present invention relates to a method and apparatus for optically inspecting a surface of a disk, and more particularly to a method and apparatus for optically detecting surface defects of a disk, such as scratches or pin holes.
Conventionally, a surface of a disk, such as a laser visual disk (which is hereinafter referred to as an LVD) or a compact disk (which is hereinafter referred to as a CD), is visually inspected and evaluated. Because of the dependency of the accuracy of visual inspection on the inspector's ability and of the low efficiency of visual inspection, much effort has been put into the development of automatic inspection technology for LVDs and CDs for many years.
To automatically detect surface defects of, for example, a long web, the web is continuously transported lengthwise in one direction at a constant speed and scanned with a flying spot in a direction perpendicular to the direction in which the web is transported. Light modulated and reflected by the surface of the web including surface defects is measured by means of a light-detecting device to detect surface defects, thereby evaluating the surface of the web. Such a method is described in, for example, Japanese Unexam. Pat. Publ. No. 55-87907. Because it is a noncontact method, it is basically effectively applicable in inspecting LVDs and CDs.
Meanwhile, optical disks, such as LVDs, have a signal recording surface that is formed thereon with a great number of pits or bumps and covered with a transparent protective layer. These pits or bumps are distributed on coaxial tracks at regular spacings. Light diffracted and reflected by the pit is detected to read a signal recorded on the recording surface of the disk. Pit configurations and arrangements are classified into two groups according to how the disks are driven to record and reproduce signals, namely the types of disk drive systems, which are a constant linear velocity (CLV) system and a constant angular velocity (CAV) system. In the CLV type of LVDs which are driven so that all tracks run at a constant and same linear velocity, the pits are formed in a same configuration and arranged at regular circumferential spacings for all tracks. On the other hand, in the CAV type of LVDs which are always driven at a constant velocity independently of the positions of the tracks, pits on a track are formed so as to be shorter in circumferential length than those on a track located outwardly of the track and are arranged more closely than those on the other track.
One such conventional surface defect inspection apparatus that scans a surface of an LVD with a flying spot is shown in FIG. A scanning beam 3 generated by a scanner 2 scans a surface of an LVD 4 along a line 5 disposed radially of the center of rotation 6 of the LVD 4. The light 3a reflected directly from the surface of the LVD 4 is received by a light-detecting unit 7 and converted into a photoelectric output thereby. The photoelectric output is analyzed to evaluate the surface of the LVD 4. Because of the uniformity of surface reflectance over the surface of the LVD 4 to be inspected, if the scanning line 5 is disposed radially of the center of rotation 6 of the LVD, the reflected light 3a from the rotating LVD has no fluctuations. If the scanning beam 3 passing through a transparent protective layer 8 coated over the LVD 4 is diffracted and diffused by the pits of the LVD 4, the rays diffracted and diffused by the pits usually travel in directions greatly different from that in which the reflected light 3a from the surface of the transparent layer 8 of the LVD 4 travels and so are not received by the light-detecting unit 7.
If the light-detecting unit 7 receives not only diffused rays 3b reflected by a surface defect S of the LVD 4 but also diffracted and diffused rays 11 reflected by the pit 10a of the LVD 4 as shown in FIG. 2, because, in the CLV type of LVDs, the light 11 diffused and reflected by the respective pit 10a is directed toward the light-detecting unit 7 at the same angle as the light diffused and reflected by the surface of the LVD 4, the light-detecting unit 7 will receive the diffracted and diffused light 11, which is undesirable to evaluate the surface of the LVD 4, as well as the reflected light 3a.
Because the amount or intensity of the light diffused by each pit is substantially constant independently of its location on the CLV type LVD, the light-detecting unit 7 can easily separate the diffused light 11 reflected by the pits 10a from those reflected by the surface defect S. In the CAV type LVD, because the pits on different tracks are different in size and distributed at different angular spacings, angles at which the light rays diffused and reflected by the pits on the different tracks are oriented are different according to the locations of the tracks. On the other hand, the angle of reflection of the diffused light from the surface of the LVD 4 is substantially constant over the surface of the LVD 4. The light-detecting unit 7, which has an effective light-detecting area having a width W (shown by a dotted line in FIG. 3), receives the diffused light 11 in the form of a circular arc reflected from the pits on different tracks at different locations thereof and misses them partly as is shown in FIG. 3. That is, as is shown in FIG. 4, the farther radially inward is the track on which a pit is located, the lower will be the photoelectric output from the light-detecting unit 7 receiving the diffused light from the pit. The change or difference of output from the light-detecting unit 7 due to the locations of the pits is considerably greater than that due to surface defects of the LVD 4. Therefore, it is quite difficult to electrically compensate such changes caused by the locations of pits and, accordingly, the conventional surface inspection method and apparatus is not always effectively applicable to LVDs.